This study of hormonal regulation is targeted to define the biochemical mechanisms whereby age-dependent changes perturb physiological control systems and, thus, lead to failure to maintain homeostasis in the aged. Investigations are focused on the biochemical interactions of hormones, which are mediated via cyclic AMP and cyclic GMP. Topics investigated include: 1) hormone receptors in membranes; 2) adenylate cyclase and guanylate cyclase activities; 3) phosphorylation of membrane proteins by cyclic nucleotide-dependent and -independent protein kinases; 4) regulation of protein phosphorylation by Ca2 ion and Ca2 ion-dependent regulatory protein (Calmodulin).